Previous studies indicate that the photoperiod regulates neuroendocrine-gonadal function in seasonally breeding animals in at least two operationally distinct ways. First, the photoperiod can alter pituitary LH and FSH release by changing the sensitivity of gonadotropin control centers to the negative feedback effect of gonadal steroid hormones (i.e. a steroid-dependent process). Second, the length of the day can alter pituitary gonadotropin release in the absence of adrenal and testicular steroid secretions (i.e. a steroid-independent process). Due to the central role of the pineal gland and the suprachiasmatic nuclei in the reproductive response to the photoperiod, we are examining their possible involvement in the steroid-dependent and steroid-independent response. We plan to examine the role of some of the metabolites of testosterone, specifically 5 alpha-dihydrotestosterone and estradiol, in the steroid dependent response. Possible sites of action of photic-induced changes in steroid feedback sensitivity will be studied via autoradiography, steroid receptor assays and an examination of the response of the pituitary gland to releasing hormone under different photoperiodic conditions. The overall goal of the proposed project is to develop a complete understanding of the neuroendocrine events which underlie the photoperiodic control of seasonal reproductive cycles in mammals.